This invention relates to a method for the production of phosphoric acid by acidulation of phosphate ore and to an apparatus for carrying out the method.
The use of artificial fertilizers derived from phosphoric acid has increased greatly during recent decades. While there continues to be strong worldwide need for fertilizers and phosphate chemicals, many of the high grade and easily mined phosphate rock deposits are being depleted, leaving lower grade ores having a lesser amount of recoverable bone phosphate of lime (BPL). This has necessitated the increased usage of processes, termed beneficiation, to remove gangue and associated impurities to increase the BPL content of the phosphate rock used as raw material for acidulation. However, even with these processes, the overall grade of the resulting upgraded phosphate rock is generally decreasing. Some deposits are too poor to be economically upgraded. These factors, among others, are contributing to increased costs for producing phosphoric acid in many parts of the world and the increasing difficulties of some producers to continue to be competitive.
Beneficiation of the phosphate ore involves a number of unit operations. For example, for sedimentary ore, the material is scalped and the fine cut sent to sizing equipment, such as vibrating screens, to produce a coarse product (pebble) and a finer sand-concentrate rock mixture. Each of the size fractions is washed and dewatered. The water circulates through large ponds to deposit entrained slimes. The coarse product goes to storage piles for shipment. The sand-concentrate mixture is subjected to successive steps of selective flotation to separate the two. The rejected sand is slurried with water and pumped to the mine for fill. The concentrate phosphate rock is placed on storage piles for shipment. The coarse material scalped from the ore is often near grade material which must be discarded due to the inability or excessive cost for upgrading to an acceptable grade.
It will immediately be apparent that beneficiation of phosphate ore is a costly procedure, which adds significantly to the cost of producing phosphoric acid. Additionally, beneficiation technology currently cannot sufficiently upgrade certain ores for use in existing phosphoric acid plants. There exists a need in the art for a process and an apparatus for economically extracting P.sub.2 O.sub.5 values from phosphate ore without the need for beneficiating the ore prior to acidulation. Where practical, the process and apparatus should be easily adapted to existing commercial phosphoric acid plants in order to minimize the capital investment required to commence large-scale operation.